The invention relates to a method and apparatus for preventing signal reflection of transmissions between electronic devices by employing a configurable, self-terminated driver.
As faster and more complex electrical circuits are designed, one challenge is to fit an ever increasing number of components into smaller form-factor devices. The increasing number of components also causes the price of high density circuits to increase. Therefore, it is desirable to make the most effective use of every component utilized without affecting the operation of the circuit.
Presently, electronic devices, such as semiconductor integrated circuits, are typically mounted on a circuit substrate and electrically interconnected to perform a practical function. As transmission speeds between such devices increase, it is desirable that a transmission path have evenly matched impedance. A problem that occurs if impedance is not evenly matched along a transmission path is signal reflection. Signal reflection occurs where a transmission signal crosses from a first transmission medium to a second transmission medium having different characteristic impedance. As transmission frequencies increase, the disruption of signal waveforms caused by such reflection becomes more problematic.
Typically, signal transmission without reflection is achieved by coupling a termination resistor equal to the characteristic impedance of the signal transmission line at either the receiving end, the transmitting end, or both. This technique is also commonly known as xe2x80x9cimpedance matchingxe2x80x9d. The termination resistor may be coupled to a voltage source (xe2x80x9cpulled upxe2x80x9d) or coupled to ground (pulled down).
As shown in FIG. 1, the transmission path between two devices 102 and 104, such as two semiconductor integrated circuits, is made up of a conductor 110 which has a distributed characteristic impedance Z0. However, the node 106 where such conductor couples to the device pins can be a source of signal reflection. Therefore, a termination resistor 108, with a matching impedance Z0, is coupled to the receiving end of the transmission path 106 at one end and to ground at the other end. This causes the transmitted signal to be absorbed by the termination resistor 108 at the receiving end, producing no reflection. The receiving device thus receives the transmitted signal without disruption of the signal waveforms.
In preventing signal reflection, it is essential that the resistance of the termination resistor 108 exactly match the characteristic impedance Z0 of the signal transmission path or conductor 110. Because such high accuracy is required when matching a resistor to the transmission line, terminating resistors are not typically placed within semiconductor integrated circuits. That is, since the characteristic impedance of transmission lines, or conductors, can vary from application to application, placing terminating resistors within an integrated circuit is not advisable because they may not be a good match with the transmission lines utilized. Placing termination resistors within integrated circuits also increases the cost of such devices. Therefore, termination resistors are commonly mounted outside the semiconductor integrated circuit as shown in FIG. 1.
One disadvantage of utilizing external termination resistors is that as the number of signal paths between devices increases, the number of termination resistors required also increases, requiring an increased area for their mounting. This presents a problem in achieving higher density, small form-factor devices such as computer memory modules.
Computer memory devices is one area where impedance matching is necessary but additional components are undesirable. Computer memory architectures commonly require impedance matching to minimize reflections at higher frequencies. FIG. 2 is a high-level schematic of a conventional computer memory architecture. A controller 202 manages communications with a plurality of memory modules 204. Note that, typically, communications between a controller and memory devices are carried out over a bus 206. The bus 206 consists of a plurality of individual transmission lines.
FIG. 3 provides more details as to the typical placement of termination resistors in the computer memory architecture of FIG. 2. A single memory module 204 and a single transmission line 206 is shown in FIG. 3 for simplicity. Termination resistors 210 are coupled to each transmission line or conductor 206 near the node where the transmission line couples with an electronic device 202, 212 and 214. Electronic devices typically found in memory modules include memory components to store data, such as dynamic random access memory (DRAM) 214, and buffers 212 that act as an interface between the bus and the memory devices. At each node where a transmission line couples to a device, a termination resistor 210 is necessary to avoid signal reflection. Termination resistors 210 thus add to the cost of memory modules 204 and make it more difficult to design memory modules with small form factors.